Process for preparing ethene

ABSTRACT

The present invention provides a process for the preparation of ethene by vapor phase chemical dehydration of a feed-stream comprising ethanol and optionally water and/or ethoxy ethane, said process comprising contacting a dried supported heteropolyacid catalyst in a reactor with the feed-stream having a feed temperature of at least 200° C.; wherein before the supported heteropolyacid catalyst is contacted with the feed-stream having a feed temperature of at least 200° C., the process is initiated by: (i) drying a supported heteropolyacid catalyst in a reactor under a stream of inert gas having a feed temperature of from above 100° C. to 200° C.; and (ii) contacting the dried supported heteropolyacid catalyst with an ethanol-containing vapor stream having a feed temperature of from above 100° C. to 160° C.

The present invention relates to a process for producing ethene by thevapour phase dehydration of ethanol using a heteropolyacid catalyst. Inparticular, the process of the present invention involves an initiationprocedure comprising drying of the heteropolyacid catalyst at a specificrange of temperature prior to use in an ethanol dehydration reaction.

Ethene is an important commodity chemical and monomer which hastraditionally been produced industrially by the steam or catalyticcracking of hydrocarbons derived from crude oil. However there remainsan increasing need to find alternative economically viable methods ofmaking this product. By virtue of its ready availability from thefermentation of biomass and synthesis gas based technologies, ethanol isemerging as an important potential feedstock from which ethene can bemade in the future.

The production of ethene by the vapour phase chemical dehydration ofethanol is a well-known chemical reaction which has been operatedindustrially for many years (see for example Kirk Othmer Encyclopaediaof Chemical Technology (third edition), Volume 9, pages 411 to 413).Traditionally this reaction has been carried out in the presence of anacid catalyst such as activated alumina or supported phosphoric acid.

In recent years attention has turned to finding alternative catalystshaving improved performance. This has led to the use of supportedheteropolyacid catalysts, such as those disclosed in EP1925363, whichhave the benefit of improved selectivity, productivity and reducedethane formation following the dehydration of a feedstock comprisingethanol and ethoxyethane for the production of ethene. This is desirablebecause firstly ethane is an undesirable by-product and secondly itsseparation from ethene on a large scale is both difficult and energyintensive. Related documents WO 2007/063281 and WO 2007/003899 alsodisclose modes of carrying out dehydration of oxygenate feedstocks withsupported heteropolyacid catalysts. Supported heteropolyacid catalystsmay be readily prepared using wet impregnation techniques by dissolvinga heteropolyacid in a suitable solvent to form a heteropolyacid solutionand then impregnating a suitable catalyst support with theheteropolyacid solution.

In the dehydration process, a feed typically comprising ethanol, andoptionally water and other components, is continuously fed to a reactorcontaining a bed of heteropolyacid catalyst and the productscontinuously removed. Under steady state conditions, the feed enteringthe reactor is rapidly converted near the inlet into an equilibriummixture of water, ethanol and ethoxyethane (the product of a rapid firststage dehydration of the ethanol). Such processes are typicallyconducted at elevated temperature and pressure.

Due to i) the nature of heteropolyacids; ii) the process for preparingsupported heteropolyacid catalysts; and iii) the loading of saidcatalysts into a reaction zone, the heteropolyacid component will almostcertainly be exposed to water (such as moisture in the atmosphere) underconditions at which it may become bound to the heteropolyacid component.Thus the hydration state of the heteropolyacid component of thesupported heteropolyacid catalyst prior to heating the supportedheteropolyacid catalyst will be above zero (i.e. the heteropolyacidcomponent of the supported heteropolyacid catalyst has water moleculeschemically bound thereto). Typically, the hydration state of aheteropolyacid decreases on exposure to increasing temperature; that is,the number of water molecules bound to the heteropolyacid decreases withincreasing temperature. The degree of hydration of theheteropolytungstic acid may affect the acidity of the supported catalystand hence its activity and selectivity.

WO 2011/104495 discloses a dehydration process for the preparation ofalkene using a supported heteropolyacid catalyst. That document teachesan initial catalyst drying step conducted at a temperature of at least220° C., so as to remove bound water such that at least part of theheteropolyacid component of the catalyst has a hydration state of zero,followed by a reduction in temperature under anhydrous atmosphere,before the catalyst is contacted with the reactant feedstream. WO2011/104495 teaches that the drying step advantageously leads toimproved ethane selectivity of the catalyst in the subsequent ethanoldehydration reaction.

It has thus now become desirable to dry the supported heteropolyacidcatalyst at high temperatures, typically around 240° C., as part of thestart-up procedure preceding ethanol dehydration. However, one problemthat has hitherto not been acknowledged relates to catalystdeactivation. It has been found that when the supported heteropolyacidcatalyst is dried at such high temperatures, deactivation of theheteropolyacid catalyst is exacerbated. Without being bound by anyparticular theory, this is believed to occur as a result of undesirableside reactions with high activation energies, which contribute todeactivation, becoming more prevalent as a result of the highertemperatures being used.

In particular, the number of heteropolyacid decomposition sites formedon the surface of the catalyst support is believed to increase duringthe high-temperature drying step, or such decomposition sites are‘seeded’ during the drying step and subsequently develop intodecomposition sites during the dehydration reaction. Mobility ofheteropolyacid at the surface of the support toward such ‘seed’ sites isalso believed to compound the problem.

Replacement of the catalyst in a dehydration system is labour intensive,has significant materials costs and involves temporarily shutting downwhat is likely to be a continuous process, which has detrimental impacton product output. Thus, a problem of catalyst deactivation poses aserious issue to the economic viability of ethanol dehydrationprocesses.

It has now surprisingly been found that catalyst lifetime in an ethanoldehydration reaction can be extended by performing a drying step, beforecommitting the supported heteropolyacid catalyst to the ethanoldehydration reaction, at lower temperatures than taught in the priorart. By performing the drying step over a specific range of intermediatetemperatures immediately followed by contacting with anethanol-containing vapour stream over another specific range oftemperatures as part of the start-up procedure for ethanol dehydration,productivity loss over time as a result of catalyst deactivation issignificantly reduced. Furthermore, at least in some embodiments, themaximum ethene productivity (mole/kg catalyst/hr) may also be increased.

According to the present invention, there is provided a process for thepreparation of ethene by vapour phase chemical dehydration of afeed-stream comprising ethanol and optionally water and/or ethoxyethane,said process comprising contacting a dried supported heteropolyacidcatalyst in a reactor with the feed-stream having a feed temperature ofat least 200° C.; and wherein before the supported heteropolyacidcatalyst is contacted with the feed-stream having a feed temperature ofat least 200° C., the process is initiated by:

-   -   (i) drying a supported heteropolyacid catalyst in a reactor        under a stream of inert gas having a feed temperature of from        above 100° C. to 200° C.; and    -   (ii) contacting the dried and supported heteropolyacid catalyst        with an ethanol-containing vapour stream having a feed        temperature of from above 100° C. to 160° C.

Preferably, the initiation of the ethanol dehydration process furthercomprises: (iii) ramping the feed temperature of the ethanol-containingvapour stream to at least 200° C., preferably over the course of 10minutes to 8 hours, more preferably over the course of 20 minutes to 4hours.

It has been surprisingly found that an ethanol dehydration process isparticularly advantageous when initiated by drying the supportedheteropolyacid catalyst and contacting with an ethanol-containing vapourstream at the above temperature ranges. In particular, this initiationprocedure reduces the level of heteropolyacid catalyst deactivationobserved in the ethanol dehydration reaction and, at least in someembodiments, increases ethene maximum productivity.

Moreover, it has been found that productivity in a process for producingethene by the vapour phase dehydration of ethanol using a heteropolyacidcatalyst is improved by operating at high temperature; in particular attemperatures higher than those exemplified in the prior art, for examplehigher than 240° C. However, it has been found that higher operatingtemperatures also exacerbate catalyst deactivation, much like the hightemperature drying steps used in start-up procedures in the prior art.Nevertheless, it has also been surprisingly found that by drying thesupported heteropolyacid catalyst and contacting with anethanol-containing vapour stream at the temperatures recitedhereinbefore, catalyst deactivation associated with subsequentlyoperating the ethanol dehydration reaction at higher than conventionaltemperatures is reduced. Thus, the drying and contacting steps (i) and(ii) which precede the ethanol dehydration reaction in accordance withthe process of the present invention, allows the full benefit of higherethene productivity to be realised whilst avoiding significant catalystdeactivation.

Mechanisms by which the supported heteropolyacids are believed toundergo deactivation during operation include: i) neutralisation byinorganic cations, such as ammonia/ammonium cations, and organicnitrogen-containing compounds; ii) carbon deposition; and iii)decomposition of the heteropolyacid to its constituent oxides.Deactivation as a result of neutralisation by inorganic cations andorganic nitrogen-containing compounds may be mitigated by committing theethanol based raw materials to a clean-up procedure to remove theneutralising species. In contrast, the features of the initiationprocedure according to the present invention are believed to largelyeliminate deactivation of the heteropolyacid catalyst as a result ofdecomposition of the heteropolyacid to its constituent oxides.

During the preparation of a supported heteropolyacid catalyst, thecatalyst may be optionally dried. However, as the skilled person willappreciate, the supported catalyst will inevitably be exposed tomoisture upon transport and introduction into a reactor. Drying removescondensed water vapour from the surface of the support which cannegatively impact the surface chemistry of the support, for instance,the acidity of the heteropolyacid component. However, drying inside thereactor in accordance with the present invention has also beensurprisingly found to reduce the formation of heteropolyaciddecomposition sites on the catalyst.

Without being bound by any particular theory, it is believed that thelow temperature drying according to step (i) of the process reduces thepossibility of forming ‘seed’ sites which may subsequently lead todecomposition. ‘Seed’ sites may be produced following, for example,changes in the oxidation state of the heteropolyacid on exposure toheat; the formation of carbon residues; and/or formation of defectstructures. Where exposure to heat has given rise to chemical changes inthe heteropolyacid at a specific surface location to form such ‘seed’sites, the likelihood of full decomposition of heteropolyacid to itsconstituent oxides at these sites is substantially increased. Moreover,mobility of heteropolyacid at the surface of the support at elevatedtemperatures towards such ‘seed’ sites exacerbates the rate ofdecomposition. As a consequence, the catalyst lifetime is significantlyextended, which has clear economic benefits relating to re-use andreplacement of the catalyst, as well as the reduction of waste. Theoperating conditions of the present invention thus correspond to anarrow window within which significant catalyst deactivation is avoided,whilst ethylene productivity is promoted.

In accordance with the present invention, the supported heteropolyacidcatalyst is dried in step (i) of the initiation under a stream of inertgas having a feed temperature of from above 100° C. to 200° C.Preferably, the supported heteropolyacid catalyst is dried in step (i)under a stream of inert gas having a feed temperature of from 100° C. to180° C.; more preferably from 110° C. to 170° C.; most preferably from120° C. to 160° C.; for example 150° C.

Reference herein to “initiated” or “initiation” in regard to steps (i)and (ii) of the process of the present invention is intended to meanthat these steps precede exposure of the catalyst to the feed-stream ata feed temperature of at least 200° C. Furthermore, “initiated” or“initiation” is also intended to mean that no other steps materiallyaffecting the composition or nature of the supported heteropolyacidcatalyst are undertaken before step (i), after the supportedheteropolyacid catalyst is positioned within the reactor.

Reference herein to “inert gas” is intended to mean a gas that is notconsumed in the reaction of the process of the present invention, and isnot consumed by any other process which may be catalysed by thesupported heteropolyacid catalyst. Examples of suitable inert gases arenitrogen, argon, helium, methane and carbon dioxide. Preferably, theinert gas is selected from nitrogen, argon and helium, more preferably,the inert gas is nitrogen. By the term “stream of inert gas” as usedherein, it is meant that the atmosphere under which the drying steptakes place is an inert gas that is constantly being removed andreplenished with fresh (or recycled) inert gas (i.e. a gas flow). Forexample, the “stream of inert gas” is preferably a stream of nitrogengas.

Reference herein to “drying” is intended to mean exposing the supportedheteropolyacid catalyst to heat such that the dew point of water vapourin the reactor, and any other vapour that may be present, is exceededunder the pressure at which the reactor is operated. The low temperaturedrying of the supported heteropolyacid as part of an initiationprocedure for an ethanol dehydration reaction in accordance with theprocess of the present invention has been found to have numerousbenefits with regard to the dominant surface chemistry of the supportedheteropolyacid catalyst.

Drying of the supported heteropolyacid catalyst in accordance with theprocess of the invention is undertaken for a period of at least onehour. Preferably, drying is undertaken for a period of from 1 to 48hours, more preferably from 2 to 16 hours, most preferably 2 to 12hours. In some embodiments, the drying time includes a period of time inwhich the feed temperature of the inert gas for drying is ramped up tomatch a higher feed temperature used for the subsequent step ofcontacting with an ethanol-containing vapour stream. Whilst not wishingto be bound by any theory, it is thought that minimizing the period ofconstant temperature, following a ramped increase of the feedtemperature of the inert gas temperature and prior to contact with theethanol-containing vapour stream, is commercially advantageous.

In accordance with the present invention, the dried supportedheteropolyacid catalyst is contacted in step (ii) of the initiation withan ethanol-containing vapour stream having a feed temperature of fromabove 100° C. to 160° C. Preferably, the dried supported heteropolyacidcatalyst is contacted in step (ii) with an ethanol-containing vapourstream having a feed temperature of from 120° C. to 158° C., morepreferably from 130° C. to 156° C., even more preferably from 140° C. to154° C., most preferably from 148° C. to 152° C., for example 150° C.

Reference herein to an “ethanol-containing vapour stream” is intended tomean a gaseous stream comprising at least 50 wt. % ethanol and thebalance being made up of diluents. Preferably, the ethanol-containingvapour stream comprises 80 wt. % or more ethanol, more preferably 90 wt.% or more; most preferably 95 wt. % or more; with the balance preferablybeing made up of inert gas diluents. Suitable inert gas diluents arenitrogen, argon, helium, methane and carbon dioxide. Preferably, theinert gas diluents are selected from nitrogen, argon and helium, morepreferably, the inert gas diluent is nitrogen. The amount of water inthe ethanol-containing vapour stream is at most 10 wt. %, preferably atmost 7 wt. %, more preferably at most 5 wt. %, even more preferably atmost 3 wt. %, and still more preferably at most 2 wt. %, based on thetotal weight of ethanol-containing vapour stream. The amount ofethoxyethane in the ethanol-containing vapour stream is at most 5 wt. %,preferably at most 3 wt. %, and more preferably at most 2 wt. %, basedon the total weight of ethanol-containing vapour stream. Mostpreferably, the ethanol-containing vapour stream is anhydrous or theethanol-containing vapour stream comprises or consists essentially ofethanol and any balance is made up of inert gas diluents. As will beappreciated, in some embodiments, the ethanol-containing vapour streammay be identical in composition to the feed-stream containing ethanolwhich undergoes ethanol dehydration. However, in preferred embodiments,the ethanol-containing vapour stream is different from the feed-streamcontaining ethanol.

Step (ii) of contacting the dried supported heteropolyacid catalyst withan ethanol-containing vapour stream has been found to be of particularbenefit in obtaining steady state conditions for the ethanol dehydrationreaction and enhancing catalyst performance. Furthermore, contacting thecatalyst with an ethanol-containing vapour stream at a temperature offrom above 100° C. to 160° C. ensures that detrimental exotherms areavoided, which can lead to undesirable competing oligomerisationreactions during the subsequent ethanol dehydration of the feed-stream.In a particularly preferred embodiment, the inert gas stream which isused for drying the supported heteropolyacid in step (i) is converted toan ethanol-containing vapour stream for contacting step (ii) by additionof ethanol vapour to the inert gas stream.

The dehydration of the feed-stream according to the present invention isbelieved (Chem. Eng Comm. 1990, 95, 27 to 39) to proceed by either thedirect dehydration to olefin(s) and water (Equation 1); or via an etherintermediate (Equations 2 and 3).EtOH

=+H₂O  (1)2EtOH

=Et₂O+H₂O  (2)Et₂O

=+EtOH  (3)

The direct conversion of the ether to two moles of olefin and water hasalso been reported (Chem. Eng. Res. and Design 1984, 62, 81 to 91). Allof the reactions shown above are typically catalysed by Lewis and/orBronsted acids. Equation 1 shows the endothermic direct elimination ofethanol to ethene and water; competing with Equation 1 are Equations 2and 3 i.e. the exothermic etherification reaction (Equation 2), and theendothermic elimination of ethoxyethane to produce ethene and ethanol(Equation 3). However, the dehydration reaction of ethanol to ethene isoverall said to be endothermic.

The present invention provides a process for the preparation of etheneby vapour phase chemical dehydration of a feed-stream comprising ethanoland optionally water and/or ethoxyethane, said process comprisingcontacting a dried supported heteropolyacid catalyst in a reactor withthe feed-stream having a feed temperature of at least 200° C.; whereinbefore the dried supported heteropolyacid catalyst is contacted with thefeed-stream having a feed temperature of at least 200° C., the processis initiated by: (i) drying a supported heteropolyacid catalyst in areactor under a stream of inert gas having a feed temperature of fromabove 100° C. to 200° C.; and (ii) contacting the dried supportedheteropolyacid catalyst with an ethanol-containing vapour stream havinga feed temperature of from above 100° C. to 160° C.

Preferably, the feed-stream comprises water and/or ethoxyethane, morepreferably the feed-stream comprises water and ethoxyethane. When bothethoxyethane and water are present in the feed-stream, it is preferredthat the molar ratio of ethoxyethane to water is from 3:1 to 1:3,preferably from 3:1 to 1:1, more preferably 2:1 to 1:1.

Suitably, the amount of water in the feed-stream of the process of thepresent invention is at most 50 wt. %, more preferably at most 20 wt. %,most preferably at most 10 wt. %, or even at most 7 wt. %, based on thetotal weight of water, ethanol and ethoxyethane in the feed-stream.Preferably, the amount of water in the feed-stream is at least 0.1 wt.%, more preferably at least 0.5 wt. % and most preferably at least 1 wt.%, based on the total weight of water, ethanol and ethoxyethane in thefeed-stream.

Suitably, the amount of ethoxyethane in the feed-stream of the processof the present invention is at most 50 wt. %, more preferably at most 40wt. %, most preferably at most 35 wt. % based on the total weight ofwater, ethanol and ethoxyethane in the feed-stream. Preferably, theamount of ethoxyethane in the feed-stream is at least 0.1 wt. %, morepreferably at least 0.5 wt. % and most preferably at least 1 wt. %,based on the total weight of water, ethanol and ethoxyethane in thefeed-stream.

The liquid product stream following olefin removal comprises mostlyunreacted ethanol, ethoxyethane and water. The applicants have foundthat it is particularly preferable to recycle the major portion of thealcohols and ethers back to the vapour phase dehydration reactor afterwater removal.

In some embodiments of the invention, the feed-stream comprises an inertgas diluent. In other embodiments, an inert gas diluent is added downthe catalyst bed, or between multiple catalyst beds arranged in seriesor in parallel, if used. Preferred diluents for the feed-stream includenitrogen, helium, ethene and/or saturated hydrocarbons, for examplehexanes, 2-methylpropane or n-butane. More preferably, the feed-streamdiluent is selected from nitrogen and/or helium.

As described above, it has now been found that higher temperatures usedfor the dehydration reaction give greater ethene productivity. Since thepresent invention diminishes the negative effects of high operatingtemperatures on catalyst deactivation, it is preferred that the driedsupported heteropolyacid is contacted with the feed-stream when it has afeed temperature of at least 220° C., more preferably at least 240° C.In particular preferred embodiments, the feed temperature is at least252° C., at least 255° C., at least 260° C., at least 280° C. or even atleast 300° C. The upper limit of the feed temperature of the feed-streamis below the temperature at which selectivity for ethene is negativelyimpacted and/or one which is overly energy intensive. Preferably, theupper limit of the feed temperature of the feed-stream is 350° C., morepreferably 325° C. Reference to “feed temperature” herein is intended torefer to the temperature of a particular stream at the point at which itis fed to the reactor.

The pressure inside the reactor during the dehydration reaction when thesupported heteropolyacid catalyst is contacted with the feed-stream ispreferably in the range of from 0.1 MPa to 4.5 MPa, more preferably at apressure in the range of from 0.5 MPa to 3.5 MPa, and most preferably ata pressure in the range of from 1.0 MPa to 2.8 MPa.

Reference herein to the pressure inside the reactor corresponds to thesum of the partial pressures of the reactants, namely those of ethanol,water and ethoxyethane, as well as the partial pressure of the etheneproduct. Unless otherwise indicated herein, partial pressures of inertgas diluents, such as helium and nitrogen, or other inert components areexcluded from the total stated pressure. Thus, reference to reactorpressure herein is in accordance with the formula:P_(reactor)=P_(water)+P_(ethanol)+P_(ethoxyethane)+P_(ethene).Furthermore, unless otherwise indicated, reference to reactor pressuresherein is to absolute pressures, and not gauge pressures.

As will be appreciated by the skilled person, there is often a pressuredrop that occurs in a dehydration reactor between the point where thefeed stream enters the reactor and that where the effluent streamemerges from the reactor. As a consequence, there is, to a varyingextent, an internal pressure gradient which exists inside the reactoritself. It is therefore to be understood that reference herein to the“pressure inside the reactor” means any pressure falling within thepressure range defined by the above-mentioned internal pressuregradient. The pressure inside the reactor itself therefore lies betweenthe feed-stream pressure and the effluent stream pressure.

The supported heteropolyacid may suitably be provided in the reactor inthe form of one or more catalyst beds in the reactor, preferablymultiple catalyst beds which may be arranged in series or in parallel.In preferred embodiments, the catalyst bed(s) is/are selected fromadiabatic packed beds, tubular fixed beds or fluid beds. Most preferablythe catalyst bed(s) in the reactor is/are selected from adiabatic packedbeds.

Desirably, the reactor is configured such that the temperaturedifferential across the one or more catalyst beds during drying of thesupported hetereopolyacid catalyst in step (i) is minimal, since thisassists with uniform drying of the supported heteropolyacid catalyst.Preferably, the temperature differential across the catalyst bed(s)during drying of the supported hetereopolyacid catalyst in step (i) isno more than 20° C., preferably no more than 15° C., more preferably nomore than 10° C., most preferably no more than 5° C. The temperaturedifferential can be readily determined by means of multiple temperaturesensors positioned at different locations across the catalyst bed.

The term “heteropolyacid”, as used herein and throughout the descriptionof the present invention, is deemed to include inter alia; alkali,alkali earth, ammonium, free acids, bulky cation salts, and/or metalsalts (where the salts may be either full or partial salts) ofheteropolyacids. Hence, the heteropolyacids used in the presentinvention are complex, high molecular weight anions comprisingoxygen-linked polyvalent metal atoms. Typically, each anion comprises12-18, oxygen-linked polyvalent metal atoms. The polyvalent metal atoms,known as peripheral atoms, surround one or more central atoms in asymmetrical manner. The peripheral atoms may be one or more ofmolybdenum, tungsten, vanadium, niobium, tantalum, or any otherpolyvalent metal. The central atoms are preferably silicon orphosphorus, but may alternatively comprise any one of a large variety ofatoms from Groups I-VIII in the Periodic Table of elements. Theseinclude copper, beryllium, zinc, cobalt, nickel, boron, aluminium,gallium, iron, cerium, arsenic, antimony, bismuth, chromium, rhodium,silicon, germanium, tin, titanium, zirconium, vanadium, sulphur,tellurium, manganese nickel, platinum, thorium, hafnium, cerium,arsenic, vanadium, antimony ions, tellurium and iodine. Suitableheteropolyacids include Keggin, Wells-Dawson and Anderson-Evans-Perloffheteropolyacids. Specific examples of suitable heteropolyacids are asfollows:

18-tungstophosphoric acid—H6[P2W18O62].xH2O

12-tungstophosphoric acid—H3[PW12O40].xH2O

12-tungstosilicic acid—H4[SiW12O40].xH2O

Cesium hydrogen tungstosilicate—Cs3H[SiW12O40].xH2O

and the free acid or partial salts of the following heteropolyacidsacids:

Monopotassium tungstophosphate—KH5[P2W18O62].xH2O

Monosodium 12-tungstosilicic—acid NaK3[SiW12O40].xH2O

Potassium tungstophosphate—K6[P2W18O62].xH2O

Ammonium molybdodiphosphate—(NH4)6 [P2Mo18O62].xH20

Potassium molybdodivanado phosphate—K5[PMoV2O40].xH2O

In addition, mixtures of different heteropolyacids and salts can beemployed. The preferred heteropolyacids for use in the process describedby the present invention is any one or more heteropolyacid that is basedon the Keggin or Wells-Dawson structures; more preferably the chosenheteropolyacid for use in the process described by the present inventionis any one or more of the following: heteropolytungstic acid (such assilicotungstic acid and phosphotungstic acid), silicomolybdic acid andphosphomolybdic acid. Most preferably, the chosen heteropolyacid for usein the process described by the present invention is any one or moresilicotungstic acid, for example 12-tungstosilicic acid(H₄[SiW₁₂O₄₀].xH₂O).

Preferably, the heteropolyacids employed according to the presentinvention may have molecular weights of more than 700 and less than8500, preferably more than 2800 and less than 6000. Such heteropolyacidsalso include dimeric complexes.

The supported catalyst may be conveniently prepared by dissolving thechosen heteropolyacid in a suitable solvent, where suitable solventsinclude polar solvents such as water, ethers, alcohols, carboxylicacids, ketones and aldehydes; distilled water and/or ethanol being themost preferable solvents. The resulting acidic solution has aheteropolyacid concentration that is preferably comprised between 10 to80 wt %, more preferably 20 to 70 wt % and most preferably 30 to 60 wt%. This said solution is then added to the chosen support (oralternatively the support is immersed in the solution). The actualvolume of acidic solution added to the support is not restricted, andhence may be enough to achieve incipient wetness or wet impregnation,where wet impregnation (i.e. preparation using an excess acidic solutionvolume relative to pore volume of support), is the preferred method forthe purposes of the present invention.

The resulting supported heteropolyacid may be modified, and varioussalts of heteropolyacid may then be formed in the aqueous solutioneither prior to, or during, impregnation of the acidic solution onto thesupport, by subjecting the supported heteropolyacid to a prolongedcontact with a solution of a suitable metallic salt or by addition ofphosphoric acid and/or other mineral acids.

When using a soluble metallic salt to modify the support, the salt istaken in the desired concentration, with the heteropolyacid solution.The support is then left to soak in the said acidic solution for asuitable duration (e.g. a few hours), optionally with periodic agitationor circulation, after which time it is filtered, using suitable means,in order to remove any excess acid.

When the salt is insoluble it is preferred to impregnate the catalystwith the HPA and then titrate with the salt precursor. This method canimprove the dispersion of the HPA salt. Other techniques such as vacuumimpregnation may also be employed.

The amount of heteropolyacid impregnated on the resulting support issuitably in the range of 10 wt % to 80 wt % and preferably 20 wt % to 50wt % based on the total weight of the heteropolyacid and the support.The weight of the catalyst on drying and the weight of the support used,may be used to obtain the weight of the acid on the support by deductingthe latter from the former, giving the catalyst loading as ‘gheteropolyacid/kg catalyst’. The catalyst loading in ‘gheteropolyacid/liter support’ can also be calculated by using the knownor measured bulk density of the support. The preferred catalytic loadingof heteropolyacid is 150 to 600 g heteropolyacid/kg Catalyst.

According to a preferred embodiment of the present invention the averageheteropolyacid loading per surface area of the dried supportedheteropolyacid catalyst is more than 0.1 micro moles/m².

It should be noted that the polyvalent oxidation states and hydrationstates of the heteropolyacids stated previously and as represented inthe typical formulae of some specific compounds only apply to the freshacid before it is impregnated onto the support, and especially before itis subjected to the dehydration process conditions. The degree ofhydration of the heteropolyacid may affect the acidity of the supportedcatalyst and hence its activity and selectivity. Thus, either or both ofthese actions of impregnation and dehydration process may change thehydration and oxidation state of the metals in the heteropolyacids, i.e.the actual catalytic species used, under the process conditions given,may not yield the hydration/oxidation states of the metals in theheteropolyacids used to impregnate the support. Naturally therefore itis to be expected that such hydration and oxidation states may also bedifferent in the spent catalysts after reaction

According to a preferred embodiment of the present invention, the amountof chloride present in/on the said heteropolyacid supported catalyst isless than 40 ppm, preferably less than 25 ppm and most preferably lessthan 20 ppm.

The supported heteropolyacid catalyst used in the process of the presentinvention may be a fresh catalyst or a previously used catalyst. Thus,in one embodiment, at least a portion of the supported heteropolyacidcatalyst has previously been employed in a process for the preparationof an ethene from a feed comprising ethanol, water and ethoxyethane. Forexample, at least a portion of the supported heteropolyacid may derivefrom an extract of heteropolyacid from a previously used catalyst i.e.from a partially deactivated material.

According to a further preferred embodiment of the present invention,the heteropolyacid supported catalyst is a heteropolytungstic acidsupported catalyst having the following characteristic:PV>0.6−0.3×[HPA loading/Surface Area of Catalyst]wherein PV is the pore volume of the dried supported heteropolytungsticacid catalyst (measured in ml/g catalyst); HPA loading is the amount ofheteropolyacid present in the dried supported heteropolyacid catalyst(measured in micro moles per gram of catalyst) and Surface Area ofCatalyst is the surface area of the dried supported heteropolytungsticacid catalyst (measured in m² per gram of catalyst).

Suitable catalyst supports may be in a powder form or alternatively maybe in a granular form, or in a pelletised form, a spherical form or asextrudates (including shaped particles) and include, but are not limitedto, clays, bentonite, diatomous earth, titania, activated carbon,aluminosilicates e.g. montmorillonite, alumina, silica-alumina,silica-titania cogels, silica-zirconia cogels, carbon coated alumina,zeolites, zinc oxide, flame pyrolysed oxides. Supports can be mixedoxides, neutral or weakly basic oxides. Silica supports are preferred,such as silica gel supports and supports produced by the flamehydrolysis of SiCl₄. Preferred supports are substantially free ofextraneous metals or elements which might adversely affect the catalyticactivity of the system. Thus, suitable silica supports are typically atleast 99% w/w pure. Impurities amount to less than 1% w/w, preferablyless than 0.60% w/w and most preferably less than 0.30% w/w. The porevolume of the support is preferably more than 0.50 ml/g and preferablymore than 0.8 ml/g.

Suitable silica supports include, but are not limited to any of thefollowing: Grace Davison Davicat® Grade 57, Grace Davison Davicat® 1252,Grace Davison Davicat® SI 1254, Fuji Silysia CariAct® Q15, Fuji SilysiaCariAct® Q10, Degussa Aerolyst® 3045 and Degussa Aerolyst® 3043.

The average diameter of the supported heteropolyacid particles ispreferably from 500 μm to 8,000 μm; more preferably from 1,000 μm to7,000 μm; even more preferably from 2,000 μm to 6,000 μm, mostpreferably from 3,000 μm to 5,000 μm. It has been surprisingly foundthat the effects of the present invention are enhanced with supportedheteropolyacid particles of larger size (i.e. falling within the aboveranges). However, in some embodiments, these particles may be crushedand sieved to smaller sizes of, for example, 50-2,000 μm, if desired.

The average pore radius (prior to impregnation with the heteropolyacid)of the support is 10 to 500 Å, preferably 30 to 350 Å, more preferably50 to 300 Å and most preferably 60 to 250 Å. The BET surface area ispreferably between 50 and 600 m²/g and is most preferably between 130and 400 m²/g.

The BET surface area, pore volume, pore size distribution and averagepore radius were determined from the nitrogen adsorption isothermdetermined at 77K using a Micromeritics TRISTAR 3000 static volumetricadsorption analyser. The procedure used was an application of BritishStandard methods BS4359: Part 1: 1984 ‘Recommendations for gasadsorption (BET) methods’ and BS7591: Part 2: 1992, ‘Porosity and poresize distribution of materials’—Method of evaluation by gas adsorption.The resulting data were reduced using the BET method (over the pressurerange 0.05-0.20 P/Po) and the Barrett, Joyner & Halenda (BJH) method(for pore diameters of 20-1000 Å) to yield the surface area and poresize distribution respectively.

Suitable references for the above data reduction methods are Brunauer,S, Emmett, P H, & Teller, E, J. Amer. Chem. Soc. 60, 309, (1938) andBarrett, E P, Joyner, L G & Halenda P P, J. Am Chem. Soc., 1951 73373-380.

Samples of the supports and catalysts were out gassed for 16 hours at120° C. under a vacuum of 5×10⁻³ Torr (0.6666 Pa) prior to analysis.

In another aspect, the present invention also provides a use of a driedsupported heteropolyacid catalyst prepared by the initiation proceduredescribed hereinbefore for improving ethene productivity and/or forextending catalyst lifetime in a process for producing ethene by thevapour phase chemical dehydration of a feed-stream comprising ethanoland optionally water and/or ethoxyethane, wherein said process comprisescontacting a supported heteropolyacid catalyst with the feed-streamhaving a feed temperature of at least 200° C.

In other aspects, the present invention also provides a compositioncomprising (or consisting of) a product obtained by a process accordingto the present invention and/or derivatives thereof, including a productobtained by a process according to the present invention per se, and/orderivatives thereof. As used herein, a derivative is a compositioncomprising or consisting of a product arising from a further process,said further process having utilised the product of the presentinvention as a feedstock at any stage. By way of non-limiting example,polyethylene may be such a derivative. As the composition/productdescribed here arises from a process as described above, any features ofthe process described above are also applicable to these aspects, eitherindividually or in any combination.

The present invention will now be illustrated by way of the followingexamples and with reference to the following figures:

FIG. 1: Graphical representation of ethene productivity against time ofcatalyst exposure to a feed stream at 260° C. for Example 1 andComparative Examples 1 to 3; and

FIG. 2: Graphical representation of ethene productivity against time ofcatalyst exposure to a feed stream at 260° C. for Example 2 andComparative Example 4.

CATALYST PREPARATION

A silicotungstic acid (STA) catalyst was used for conducting thedehydration reactions according to the following examples.

A high purity silica support with a surface area of 147 m²/g, porevolume of 0.84 ml/g and a mean pore diameter of 230 Å was used forpreparation of the STA catalyst. The catalyst was prepared by addingsilica (512 g) to a solution of silicotungstic acid (508 g) in water(1249 g). Once the silicotungstic acid solution had fully impregnatedthe pores of the support the excess solution was drained, under gravity,from the support and this was then dried.

The STA loading on the catalyst support as STA.6H₂O, on a dry weightbasis, was estimated to be 24.5% w/w, based on the weight gained by thesilica during the catalyst preparation.

For Example 1 and Comparative Examples 1 to 3 below, the catalyst wascrushed to a particle size of 100 to 200 μm before being loaded into thereactor tube.

For Example 2 and Comparative Example 4 below, the catalyst was crushedto a particle size of 850 to 1000 μm before being loaded into thereactor tube.

Vapour Phase Dehydration Reactions for Example 1 and ComparativeExamples 1 to 3

A mass of STA catalyst (as indicated in Table 1 below) prepared inaccordance with the above method was loaded into a reactor tube havingan isothermal bed and pressurised to 0.501 MPa under inert gas (nitrogenand helium) flow. The catalyst was heated at 2° C./min to either 150° C.(Example 1) or 240° C. (Comparative Examples 1 to 3) under a combinednitrogen (0.01500 mol/hr) and helium flow (0.00107 mol/hr) and held atthis temperature for 8 hours before being cooled to 150° C., if notalready at this temperature.

Ethanol (0.04084 mol/hr) was then added to the nitrogen/helium flow andthe temperature was increased at 2° C./min to 225° C. Once at 225° C.the feed pressure was increased at a rate of 0.1 MPa/min such that thepressure inside the reactor was increased to the value of 2.858 MPa. Thediethyl ether and water reagents were added to the ethanol, helium andnitrogen flow. At this point the flows of the feed components wereadjusted to give ethanol (0.02677 mol/hr), diethyl ether (0.00776mol/hr), water (0.00297 mol/hr), helium (0.00106 mol/hr) and nitrogen(0.01479 mol/hr).

Once the catalyst performance had stabilised at 225° C., typically afteraround 100 hrs, the catalyst temperature, which is the same as the feedtemperature in this particular reactor, was increased to 260° C. and theethene productivity monitored versus time by on-line GC analysis. Theresults of dehydration experiments at varying pressure are presented inTable 1 below.

TABLE 1 Max. Time on Ethylene Temp. Mass Stream Pro- Total Pro- under ofat cess Pres- ductivity N₂ flow catalyst 260° C. Temp. sure (mole/kgExample (° C.) (mg) (hrs) (° C.) (MPa) catalyst/hr) Example 1 150 27.24.11 260 2.858 478 Example 1 150 27.2 37.8 260 2.858 397 Example 1 15027.2 72.47 260 2.858 379 Example 1 150 27.2 139.84 260 2.858 281 Example1 150 27.2 186.99 260 2.858 268 Example 1 150 27.2 207.19 260 2.858 263Example 1 150 27.2 254.33 260 2.858 215 Comparative 240 27.16 0.84 2602.858 411 Example 1 Comparative 240 27.16 27.97 260 2.858 314 Example 1Comparative 240 27.16 48.14 260 2.858 271 Example 1 Comparative 24027.16 81.94 260 2.858 239 Example 1 Comparative 240 27.16 115.66 2602.858 164 Example 1 Comparative 240 27.16 183.1 260 2.858 9 Example 1Comparative 240 27.16 230.26 260 2.858 5 Example 1 Comparative 240 27.16250.43 260 2.858 5 Example 1 Comparative 240 27.3 5.36 260 2.858 397Example 2 Comparative 240 27.3 38.97 260 2.858 344 Example 2 Comparative240 27.3 72.67 260 2.858 257 Example 2 Comparative 240 27.3 139.89 2602.858 210 Example 2 Comparative 240 27.3 186.95 260 2.858 101 Example 2Comparative 240 27.1 5.77 260 2.858 342 Example 3 Comparative 240 27.132.67 260 2.858 287 Example 3 Comparative 240 27.1 66.29 260 2.858 232Example 3 Comparative 240 27.1 140.99 260 2.858 154 Example 3Comparative 240 27.1 200.38 260 2.858 30 Example 3 Comparative 240 27.1257.72 260 2.858 4 Example 3

The results in Table 1, which are represented graphically in FIG. 1,illustrate the benefits of the process of the invention with regard tocatalyst lifetime. It is clear from FIG. 1 that ethene productivityremains high with Example 1, which benefits from having had a catalystdrying step at a temperature of 150° C. in accordance with theinvention, for a significantly longer period of time compared withComparative Examples 1 to 3, which have had a catalyst drying step athigh temperature (240° C.) not in accordance with the present invention.FIG. 1 also illustrates that the maximum ethene productivity in theethanol dehydration reaction may also be increased by virtue of a dryingstep according to the present invention, in comparison with a hightemperature drying step not in accordance with the invention as in thecase of Comparative Examples 1 to 3. The maximum ethene productivityobserved for Example 1 was 478 mole/kg catalyst/hr, whilst the maximumethene productivity observed for Comparative Examples 1 to 3 was only411 mole/kg catalyst/hr (Comparative Example 1).

Vapour Phase Dehydration Reactions for Example 2 and Comparative Example4

A mass of STA catalyst (as indicated in Table 2 below) prepared inaccordance with the above method described above was loaded into areactor tube and pressurised to 0.5 MPa under nitrogen flow. Thecatalyst was heated to either 150° C. or 240° C. under nitrogen (0.4957mol/hr) flow and held at this temperature for 2 hours before beingcooled to 150° C., if not already at this temperature.

Ethanol (1.3228 mol/hr) was then added to the nitrogen flow and thetemperature of the feed to the catalyst bed was increased to 225° C.Once at 225° C. the feed pressure was increased to the value of 2.857MPa. The diethyl ether and water reagents were then added to the ethanoland nitrogen flow. At this point the flows of the feed components wereadjusted to give ethanol (0.8544 mol/hr), diethyl ether (0.2476 mol/hr),water (0.0949 mol/hr) and nitrogen (0.4957 mol/hr).

After 24 hrs the temperature of the feed to the catalyst bed wasincreased to 260° C. and the ethylene productivity monitored versus timeby on-line GC analysis. The results of dehydration experiments atvarying pressure are presented in Table 2 below.

TABLE 2 Max. Time on Ethylene Temp. Mass Stream Pro- Total Pro- under ofat cess Pres- ductivity N₂ flow catalyst 260° C. Temp. sure (mole/kgExample (° C.) (g) (hrs) (° C.) (MPa) catalyst/hr) Example 2 150 0.507 0260 2.857 444 Example 2 150 0.507 15 260 2.857 427 Example 2 150 0.50729 260 2.857 424 Example 2 150 0.507 45 260 2.857 413 Example 2 1500.507 59 260 2.857 415 Example 2 150 0.507 89 260 2.857 402 Example 2150 0.507 115 260 2.857 411 Example 2 150 0.507 130 260 2.857 415Example 2 150 0.507 145 260 2.857 410 Example 2 150 0.507 160 260 2.857395 Example 2 150 0.507 175 260 2.857 410 Example 2 150 0.507 190 2602.857 404 Example 2 150 0.507 206 260 2.857 401 Comparative 240 0.508 0260 2.857 436 Example 4 Comparative 240 0.508 16 260 2.857 402 Example 4Comparative 240 0.508 30 260 2.857 398 Example 4 Comparative 240 0.50845 260 2.857 383 Example 4 Comparative 240 0.508 60 260 2.857 376Example 4 Comparative 240 0.508 115 260 2.857 335 Example 4 Comparative240 0.508 130 260 2.857 334 Example 4 Comparative 240 0.508 145 2602.857 330 Example 4 Comparative 240 0.508 160 260 2.857 319 Example 4Comparative 240 0.508 176 260 2.857 316 Example 4 Comparative 240 0.508191 260 2.857 300 Example 4 Comparative 240 0.508 206 260 2.857 287Example 4

The results in Table 2, which are represented graphically in FIG. 2,illustrate the benefits of the process of the invention with regard tocatalyst lifetime. It is clear from FIG. 2 that ethene productivityremains high with Example 2, which benefits from having had a catalystdrying step at a temperature of 150° C. in accordance with theinvention, for a significantly longer period of time compared withComparative Example 4, which has had a catalyst drying step at hightemperature (240° C.) not in accordance with the present invention. Itwill also be appreciated that ethene productivity is generally higherfor Example 2 than for Example 1. For instance, even after 206 hours onstream, the ethene productivity for Example 2 is above 401 mole/kgcatalyst/hr, whereas after 207 hours on stream the ethene productivityfor Example 2 is above 263 mole/kg catalyst/hr.

The dimensions and values disclosed herein are not to be understood asbeing strictly limited to the exact numerical values recited. Instead,unless otherwise specified, each such dimension is intended to mean boththe recited value and a functionally equivalent range surrounding thatvalue. For example, a dimension disclosed as “40 mm” is intended to mean“about 40 mm.”

Every document cited herein, including any cross referenced or relatedpatent or application, is hereby incorporated herein by reference in itsentirety unless expressly excluded or otherwise limited. The citation ofany document is not an admission that it is prior art with respect toany invention disclosed or claimed herein or that it alone, or in anycombination with any other reference or references, teaches, suggests ordiscloses any such invention. Further, to the extent that any meaning ordefinition of a term in this document conflicts with any meaning ordefinition of the same term in a document incorporated by reference, themeaning or definition assigned to that term in this document shallgovern.

While particular embodiments of the present invention have beenillustrated and described, it would be obvious to those skilled in theart that various other changes and modifications can be made withoutdeparting from the spirit and scope of the invention. It is thereforeintended to cover in the appended claims all such changes andmodifications that are within the scope and spirit of this invention.

The invention claimed is:
 1. A process for the preparation of ethene byvapour phase chemical dehydration of a feed-stream comprising ethanoland optionally water and/or ethoxyethane, said process comprisingcontacting a dried supported heteropolyacid catalyst in a reactor withthe feed-stream having a feed temperature of at least 200° C.; whereinbefore the supported heteropolyacid catalyst is contacted with thefeed-stream having a feed temperature of at least 200° C., the processis initiated by: (i) drying a supported heteropolyacid catalyst in areactor under a stream of inert gas having a feed temperature of fromabove 100° C. to 200° C. to form a dried supported heteropolyacidcatalyst; and (ii) contacting the dried supported heteropolyacidcatalyst with an ethanol-containing vapour stream having a feedtemperature of from above 100° C. to 160° C.
 2. The process according toclaim 1, wherein the supported heteropolyacid catalyst is dried in step(i) under a stream of inert gas having a feed temperature of from 100°C. to 180° C.
 3. The process according to claim 1, wherein the driedsupported heteropolyacid catalyst is contacted in step (ii) with anethanol-containing vapour stream having a feed temperature of from 120°C. to 158° C.
 4. The process according to claim 1, wherein the feedtemperature of the feed-stream is at least 220° C.
 5. The processaccording to claim 1, wherein the upper limit of the feed temperature ofthe feed-stream is 350° C.
 6. The process according to claim 1, whereinthe pressure inside the reactor when the supported heteropolyacidcatalyst is contacted with the feed-stream is from 0.1 MPa to 4.5 MPa.7. A process according to claim 1, wherein the initiation of the ethanoldehydration process further comprises: (iii) ramping the feedtemperature of the ethanol-containing vapour stream to at least 200° C.8. The process according to claim 1, wherein the feed-stream compriseswater and/or ethoxyethane.
 9. The process according to claim 1, whereinthe ethanol-containing vapour stream comprises or consists of ethanol,any balance being made up of inert gas diluents.
 10. The processaccording to claim 1, wherein drying in step (i) is undertaken for aperiod of from 1 to 48 hours.
 11. The process according to claim 1,wherein the catalyst is provided in the form of one or more catalystbeds in the reactor.
 12. The process according to claim 11, wherein thecatalyst is provided in the form of multiple catalyst beds arranged inseries or in parallel.
 13. The process according to claim 11, whereinthe catalyst bed(s) is/are selected from adiabatic packed beds, tubularfixed beds or fluid beds.
 14. The process according to claim 11, whereinthe temperature differential across the bed of supported hetereopolyacidcatalyst in the reactor during drying of the supported hetereopolyacidcatalyst in step (i) is no more than 20° C.
 15. The process according toclaim 1, wherein the average diameter of the supported heteropolyacidcatalyst particles is from 500 μm to 8,000 μm.
 16. The process accordingto claim 1, wherein the amount of heteropolyacid in the supportedheteropolyacid catalyst is in the range of from 10 wt. % to 50 wt. %based on the total weight of the supported heteropolyacid catalyst. 17.The process according to claim 1, wherein at least a portion of thesupported heteropolyacid catalyst has previously been employed in aprocess for the preparation of an ethene from a feed-stream comprisingethanol, water and ethoxyethane.
 18. The process according to claim 1,wherein the supported heteropolyacid catalyst is a supportedheteropolytungstic acid catalyst.
 19. The process according to claim 1,wherein the feed temperature of the feed-stream is at least 220° C. andthe upper limit of the feed temperature of the feed-stream is 325° C.20. A process according to claim 1, wherein the pressure inside thereactor is from 1.0 MPa to 2.8 MPa.